In a review of almost 100 past studies covering nearly three million people, researchers found that being overweight or slightly obese was linked to about a 6 per cent lower risk of dying, compared to people considered "normal weight."

Being severely obese, however, was still tied to an almost 30 per cent higher risk of death.

The idea that being somewhat overweight could be linked to better health has been dubbed the obesity paradox, even though actual obesity is generally not associated with the apparent "benefit".

"This is actually the common finding," said the new study's lead author Katherine Flegal, a senior scientist from the US Centers for Disease Control and Prevention in Hyattsville, Maryland.

Her work, she said, confirms what previous analyses found - a link between being somewhat overweight and having a lower risk of death.

The paradox, as scientists have called it, is based on past findings that suggest overweight and obese people - even those with additional health problems - live longer than their thinner counterparts.

Some have argued that the pattern is a statistical one only because being thin, especially in old age, is often a sign or a result of serious illness - so the thinner people seem to have higher mortality.

The study results certainly do not give people permission to pack on extra pounds, according to Dr. Steven Heymsfield, the executive director of the Pennington Biomedical Research Center in Baton Rouge, Louisiana.

Heymsfield, who co-authored an editorial accompanying the new report, said the difference in mortality between overweight and normal weight people is probably very small.

"That's actually a very small number. It's probably only statistically significant because of the large number she had in her study," he added.

Also, there are concerns that body mass index (BMI) - a measurement of weight in relation to height - is not an accurate measure of someone's health risks.

For example, Heymsfield said a soldier may be considered overweight but still be healthy, because he or she has more muscle mass.

"It's not a good marker for body fat or health risk," he said.

There is also confusion around what BMI should be considered "normal," which is why Flegal and her colleagues conducted their analysis.

Past studies looking at the link between BMI and death used varying ranges to describe normal weight, overweight and obesity.

"There seems to be a lot of confusion about this whole area, and part of the confusion is that people are using a bunch of different categories," said Flegal.

For the new analysis, the researchers, who published their results in the Journal of the American Medical Association, used data from past studies, and classified the risks according to BMI categories accepted by the World Health Organization and the US National Heart, Lung, and Blood Institute.

Those organizations consider a BMI between 18.5 and 24.9 normal weight, between 25 and 29.9 overweight, and 30 or above obese. They further subdivide the obese category, though - with a BMI between 30 and 34.9 designated grade-1 obesity, and anything above 35 grades 2 or 3.

The researchers reviewed databases of medical research and found 97 studies that looked at weight and mortality risk. Combined, Flegal and her colleagues had information on about 2.9 million people from around the world and 270,000 deaths.

Being obese - in general - was linked to an 18 percent greater risk of death, compared to being normal weight. Being severely obese - grades 2 and 3 - was linked to a 29 percent greater risk of death.

However, being merely overweight was linked to a 6 percent decreased risk of death compared to a normal weight person, while being slightly (grade 1) obese was linked to a 5 percent lower risk.

The study cannot say why there seems to be a link between being overweight or slightly obese and a lower risk of death.

"We don't have the data to look at the physiological mechanisms, and that wasn't our goal," said Flegal.

"Our contribution - I hope - is just to summarize it to show what other articles are showing," she said.

For Heymsfield, the findings reinforce the common belief that increased weight is tied to an increased risk of death, but highlights the fact that someone classified as "overweight" is not necessarily "very unhealthy."

On the other hand, he warns, important markers of health, such as blood pressure and cholesterol, do respond to minor changes in weight.

"So gaining that extra 10 or 20 pounds can put you into a dangerous category, and it's important to find out if you're one of those people," he said.